Sealed heating systems.
If your external expansion vessel has a brass fitting similar to this, then be forever grateful to the installer!
The expansion vessel is an essential part of any sealed heating system, and unfortunately, too many heating engineers simply overlook this.
It should be checked annually, and repressurised where necessary.
With this valve, the whole testing procedure takes less than 5 minutes. Without it? Potentially several hours...that's why it often doesn't get checked!
Most engineers don't install these valves. Is it because they don't appreciate it's function, or purely for cost savings? It doesn't make sense!
We ALWAYS install these valves. For the sake of £20, it's false economy not to!